The use of dialysis to treat patients with kidney disease is well-known. The treatment involves the use of an artificial kidney dialyzer which is a device comprising a first compartment for the flow of blood to be dialyzed and a second compartment for the flow of an aqueous dialysis fluid (or "dialysate" as it is sometimes called). The two compartments are separated from one another in the device by a semipermeable membrane suitable for the dialysis procedure. Such semipermeable membranes are commercially available and are made from, for example, regenerated cuprammonium cellulose or cellulose acetate. The semipermeable membranes may be used in kidney dialyzers in the form of sheet, tubing or hollow fibers.
In its broadest aspect, hemodialysis involves withdrawing blood from a patient and passing that blood through the blood flow compartment of the artificial kidney while at the same time passing aqueous dialysis fluid through the dialysate compartment. As the blood flows through the dialyzer, impurities such as urea and creatinine are transported through the semipermeable membrane and are dissolved in the dialysate. Cleansed blood exiting the dialyzer is returned to the patient, while the dialysate containing the impurities removed from the blood is recirculated or discarded.
The dialysate comprises an aqueous solution of electrolytes which is prepared, either on a batch basis or continuously, by dissolving the electrolytes in water or by diluting a concentrated aqueous solution of the electrolytes (called a "dialysate concentrate") with water. In either case, the "standard" dialysate customarily used to carry out the dialysis treatment has a fixed composition which typically comprises about 136 milliequivalents per liter (meq./l.) of sodium ion, about 3.5 meq./l. of calcium ion, about 1.5 meq./l. of magnesium ion, about 2.6 meq./l. of potassium ion, about 106.6 meq./l. of chloride ion, and about 37 meq/l. of acetate ion. (In some instances, part or all of the acetate ion may be replaced by bicarbonate ion).
Since, as is well known, the conductivity of an aqueous solution of electrolytes is a function of the concentration of electrolytes dissolved therein, it is possible to ascertain whether the desired concentration of electrolytes is present in the dialysate being supplied to the dialysate compartment by measuring the conductivity of the dialysate. Thus a hemodialysis system typically comprises a conductivity cell which is placed in the dialysate line between the source of dialysate and the inlet to the dialysate compartment and which continuously monitors the conductivity of the entering dialysate. The conductivity cell has three electrodes uniformly spaced in an epoxy casing. Two of these electrodes are wired together internally and exit the cell body at a "common" terminal; the third of the electrodes exits the cell at a "signal" terminal. The conductivity cell is part of a conductivity monitor circuit which is designed to create a small voltage between the cell's "signal" and "common" terminal.
The amount of the flow of electrons which results from this voltage will depend upon the conductivity of the dialysate solution flowing through the conductivity cell. In the event the measured conductivity of the dialysate is more than a fixed amount, e.g. 5%, above or below the desired conductivity, the monitor circuit automatically sends a signal to its associated logic circuitry which, in turn, produces an alarm (either audible or visual or both) indicating that the dialysate conductivity limits have been exceeded. Since conductivity is also temperature dependent it is common practice to include a thermistor in the conductivity circuit. This thermistor, which is located on the inlet side of the conductivity cell, continuously feeds dialysate temperature information to the conductivity monitor circuit, thus allowing that circuit to compensate for any changes in dialysate temperature. Thus it will be seen that the conductivity cell measures the dialysate conductivity on a continuous basis and sounds an alarm if that conductivity deviates more or less than a fixed amount from a constant conductivity value.